The present invention relates to a magnetic bubble memory device and particularly to a high density and large capacity bubble memory device.
Bubble memory devices using ion-implanted tracks, which replace the conventional memory devices using permalloy tracks, have been developed to realize high density devices. In the ion-implanted devices, as is disclosed in U.S. Pat. No. 3,828,329, ion-implanted tracks are fabricated by implantation of ions such as H.sub.2.sup.+, He.sup.+ or Ne.sup.+ into garnet films. Permalloy tracks have poor bias margin for short period, such as 4 .mu.m or less, tracks. On the other hand, ion-implanted tracks have enough bias margin in case that the period of tracks is larger than 3 or 4 times of bubble diameter.
However, bubble control functions, fabricated by ion-implanted tracks and conductor patterns, have poor bias margin for a replicate gate and a swap gate where functions for a generator and a detector has enough bias margin. Hence, a hybrid bubble memory device, in which greater part of minor loops are composed of ion-implantation tracks and some part of minor loops and functions which are arranged both sides of minor loops, has been proposed. The hybrid device, as is disclosed in U.S. Pat. No. 4,528,645, utilize the relaxed permalloy tracks which have period 3 to 6 times as large as ion-implantation tracks, which improve characteristics of functions. An architecture of bubble memory devices using ion-implanted tracks for greater part of minor loops and permalloy tracks for some part of minor loops and functions which include a read major line 11, a write major line 12, a replicate gate 9, a swap gate 10, a generator 14 and a detector 13, is shown in FIG. 1. Region 2 of the minor loops is composed of ion-implanted tracks and region 3 and 4 of the minor loops are composed of permalloy tracks. The numbers 5, 6, 7 and 8 respectively denote the junctions between ion-implanted tracks and permalloy tracks. The nearly same design as conventional devices using permalloy tracks is applied to a replicate gate 9, a swap gate 10, a read major line 11, a write major line 12, a detector 13 and a generator 14. Minor loops using a folded structure, as is shown in FIG. 1, enables to relax the permalloy tracks' period in region 3 and 4, and the permalloy patterns included in junctions 5 to 8 and functions 9 to 14. The folded structure of minor loops is essential to realize a hybrid bubble memory device having good characteristics.
As for bubble memory devices using functions composed of only ion-implanted tracks and conductor patterns, relaxed ion-implanted tracks, having period 3 to 6 times as large as tracks for minor loop data storage area, are necessary to improve characteristics of gates. Therefore, for this device, the folded structure of minor loops is inevitable. For folded minor loops, corners 15 and 16 in which the bubble propagation direction is changed by 180 degree are provided. At an turn 15, as is shown in FIG. 2, the boundary of ion-implanted region 17 and non-implanted region 18 convex to ion-implanted region 17. Therefore, charged walls, which attract and propagate bubbles, are relatively easily formed. Enough bias field margin as large as that for straight line propagation is obtained for turns with the drive field amplitude down to 40 Oe. However, the inside turn 16 is surrounded by non-implanted region in 3 directions which have angle of 90 degree to each other. The inside turn is, specifically defined by the ion-implanted tracks where bubble propagation direction is changed by 180 degree and the implanted region surrounded by non-implanted region in 3 directions which have angle of 90 degree to each other and by implanted region in the remaining direction. Such structure of inside turns gives a bad effect on a formation of charged walls with drive field less than 50 Oe. The inside turn, as is shown in FIG. 3 and as is disclosed in BSTJ Vol. 59, No. 2, pp. 229 to 257, is composed of three cusps 19, 20 and 21 and two tips 22 and 23. This inside turn has been utilized for a memory device which has 4 .mu.m period straight line tracks in minor loops and 1 .mu.m diameter bubbles. The bias margin for the inside turn is the same as the straight line, which means good characteristics. However, when the ion-implanted tracks period is shrinked to 3 .mu.m and bubble diameter to 0.9 .mu.m, the bubble propagation characteristics of the inside turn is very poor. Operating region of the bubble propagation, which is defined by bias field and drive field is shown in FIG. 4. For 3 .mu.m period straight tracks, bubbles are stably propagated with the condition as shown by the area inside the curve 24. On the other hand, inside turn has very narrow operating region as shown by the area inside the curve 25. Relative bias margin value, which is defined by the bias margin width over the mean bias field value of the upper end and lower end, is larger than 10% for straight line. Hence, inside turn has only 2% relative bias field margin or 60 Oe drive field and bubbles cannot propagate the turn with device field less than 50 Oe.
As a result of investigating the bubble propagation errors at the inside turns, it turns out that the magnetic pole strength is weak at the tip 23 and the cusp 20 as is shown in FIG. 3. The charged wall is a kind of magnetic wall having magnetic charge which is formed by oppositely magnetized ion-implanted layers near the boundary of ion-implanted region 17 and non-implanted region 18.
The magnetization of the ion-implanted layer tends to be aligned in the easily magnetized directions 27, 28 and 29 shown in FIG. 5 because of the 120.degree.-symmetry of magnetic garnet film. In order to generate the charged wall near the cusp 20, the magnetization of the area near it must be aligned in the directions 30 and 31 shown in FIG. 5. However, both of these directions 30 and 31 are the directions in which magnetization is hard to align. The directions 30 and 31 are opposite to easily magnetized directions 28, 29 respectively. Hence, the magnetic pole of the charged wall is weak in the area near the cusp 20.
Similarly, the charged wall at the tip 23 is generated by aligning the magnetization near the tip in the directions 32 and 33 shown in FIG. 6. The direction 32 is opposite to the easily magnetized direction 28 and the direction 33 is substantially opposite to the easily magnetized direction 27. These two directions 32 and 33 are the directions hard to magnetize. As a result, at the tip 23, the magnetic pole of the charged wall is weak,
Consequently, in the inside turn composed of the two tips 22 and 23 and the three cusps 19, 20 and 21, as shown in FIG. 3, the bias field margin of magnetic bubble propagation is very poor.